AirSpace Blog's post tells the story of famed aircraft modeler Frank Ehling and his tiny, fly powered aircrafts. Currently in the Smithsonion, these are the smallest planes the museum owns. What's most interesting, however, is AirSpace Blog's description of Ehling's fly harnessing process:

Says the AirSpace Blog,

The Washington Post's 2001 obituary of Ehling described the procedure for procuring the flies: "…Ehling honed an effective technique involving cupping a fly with his hands and then hurling it to the ground to knock it unconscious. He would then dab glue on its rear end, carefully avoiding its delicate wings, and attach the fly to the plane. He also was known to capture the fly, stick it in the freezer and glue it to the wood while it was immobile from the cold.

"Either way — as the fly gained consciousness or returned to room temperature — the winged insect would lift the model plane into the air."

Theoretically, when the fly tired from its effort to stay airborne with the additional weight and drag of the airplane, the model would then glide to the ground.

Interested in trying it yourself? This video from the fly powered aircraft inventor himself, Thomas Fetterman, is pretty informative: